text:
Can a MEI CFI give a valid flight review in a plane for which he is not endorsed?
A friend of mine is working towards his initial CFI.  I'm helping him work through regulatory, weather, and other knowledge stuff when time permits.  During our most recent ground session he asked me a question that I had never been asked.  Here's the setup...
A CFI with a freshly minted MEI (airplane, land) is asked to conduct a flight review for a customer in the customer's own Cessna 421C, a pressurized piston twin certified to >30,000'.  This CFI has only flown one type of twin (pick a common training twin, doesn't really matter).  He has no time at all in a 421 of any sort or any other twin aside from the trainer in which he trained for his commercial-multi and MEI.  Our MEI is otherwise current in all categories and classes for which he is certified, holds a complex endorsement gained during his commercial training, and holds a First Class with no limitations.
The customer is current in all categories and classes for which he is certified, is within the 24mo flight review window, and has a First Class.  In short, the customer is fully qualified to be PIC for the flight portion of the flight review.
What's obvious:

The five hour rule does not apply since a flight review is not
instruction given for a certificate or rating.
Our MEI cannot act as PIC of the customer's 421C because the MEI is missing a high altitude endorsement and a high performance endorsement even though he is category and class certified for airplane multi land.
A CFI does not need to be PIC, or even hold a medical, in cases where the student/customer is able to act as PIC.
Edit: it's a really bad idea to do fly an aircraft type with which you are not familiar.

The question is this:
Can our fresh MEI give the customer a valid flight review in the customer's 421C?

The flight instructor is limited to give training in the category and class ratings of their pilot and instructor certificates, not by make and model.
Per §61.193 and §61.195 the flight instructor is authorized to train and issue endorsements for a flight review so long she or he holds a pilot certificate and flight instructor certificate with the applicable category and class rating.
Furthermore, per §61.51(e)(3), the flight instructor may log pilot in command flight time for all flight time while serving as the authorized instructor in an operation if the instructor is rated to act as pilot in command of that aircraft.
The instructor is advised according to Advisory Circular: AC 61-98C,  Section 4-2 Paragraph e. Instructor Qualifications:

Instructors should also consider their own experience and
  qualifications in a given make and model aircraft prior to giving a review in that model... ...To conduct a flight review in a multiengine
  airplane, the instructor must hold an airplane multiengine rating on their pilot and flight
  instructor certificates. For aircraft in which the CFI is not current or with which he or she is not
  familiar, he or she must obtain recent flight experience or sufficient knowledge of aircraft
  limitations, characteristics, and performance before conducting the review. In any case, the CFI
  must observe the rating limitations of § 61.195.

So, yes.  An appropriately rated flight instructor (whether or not they should) is authorized to conduct the flight review.
What's more, even if the instructor is not qualified to act as PIC (e.g. does not have a high-performance training endorsement), that instructor will still log pilot-in-command time as instructor because he or she is appropriately rated to do so.

I wouldn't necessarily write off the value of a freshly minted instructor giving a crusty old dog a flight review.  There should be learning both ways.

Agree.  Tech is the new thing.  I wouldn't expect an older pilot flying a stwam 175 to know all about WAAS.  Could be a good lesson for both.

I'm not sure I see how the instructor can log PIC. 61.51(e)(iii) applies to someone acting as PIC in an aircraft or operation that requires more than one pilot. But the instructor can't act as PIC and the operation doesn't require more than one pilot. Did I miss something? That doesn't change the rest of your answer, though.

@Pondlife, you referred to AOPA in your answer.  This is a quote from AOPA, "Flight instructors may log any flight time as PIC whenever they are providing flight instruction, whether or not they are acting as PIC (FAR 61.51)."

The instructor must only be rated for the category and class.  §61.51(e)(3) says, A certificated flight instructor may log pilot in command flight time for all flight time while serving as the authorized instructor in an operation if the instructor is rated to act as pilot in command of that aircraft.

@STWilson Ah, got it, thanks! I was looking in the wrong part of 61.51.

The rules are complex and difficult to read, especially when not indented!

To clarify, this question has nothing to do with makes and models of planes OR logging PIC.  The question is, "can a CFI give a valid BFR in a plane with characteristics for which the CFI himself is not endorsed?"  I believe the answer is YES because 61.56 only calls out category and class requirements.  I think STWilson's answer touches on that.  I've asked my friend to draft a letter to FAA chief legal council if he really wants a black and white answer.

I may be oversimplifying, but it looks to me like the main question here is just "can an instructor give a flight review without acting as PIC?". You've given one specific example of why the instructor can't act as PIC, but there could be others.
The short answer is that as long as the pilot can legally act as PIC, the instructor doesn't have to. AOPA has a nice guide that gives this advice for instructors:

35. Who acts as pilot in command during a flight review?
  This question should be resolved before the flight so that both you and the
  pilot have a clear understanding of PIC responsibilities. You should
  inspect the pilot’s logbook, pilot certificate, and medical
  certificate to ensure that he or she is qualified to act as pilot in
  command. If the pilot does not meet the pilot-in-command requirements,
  you must assume that role. While you are in the process of inspecting
  paperwork, don’t forget to check the status of the aircraft. Is it
  airworthy?

There's also a secondary question of whether it's a good idea for an instructor to give a flight review in an aircraft that he isn't familiar with. STWilson's answer covers that nicely.

In a flight review, the instructor is supposed to evaluate the pilot flying simulated IMC, which may also include unusual attitude recoveries.  The point is that the pilot being evaluated is under the "hood" and the CFI would need to be capable of acting as PIC.  This means a current medical as well as any aircraft requirements.
As near as I can determine the only loophole possibility is if the flight were conducted under IFR, in which case the CFI would not be needed as a crew member because of the flight rules.
So lack of medical (and the requirement to have one to act as PIC during hooded pilot operations), combined with high altitude endorsement, time in the C421, all work to make the administration of a flight review a no-go.
Tangentially, a safety pilot for IFR practice must also have a valid medical.


text:
Shouldn't the upwash on an airfoil increase the effective angle of attack?
From what I understand, induced drag comes from a reduction of the effective angle seen by the airfoil.
However, from wind tunnel smoke visualization, we can see that the streamlines at the leading  edges and stagnation point tend to "go upward'. Which is the opposite of what the effective relative wind is supposed to be and do.
Is this "upward tilt angle" of the streamlines, the result of suction from the upper surface minus the induced angle?
I've included two pictures:

The problem is the true AOA varies depending on where in the flow you measure it.  So what point do you use?  An inch forward of the LE?  Farther forward?  Higher? Lower?
Better to use a consistent value that is independent of the local flow variations in the upwash by using the angle of the chord line to the free stream.

so my assumption is correct?

Yes, but the value you get is not of much use unless you only need to know the local flow direction at the measured spot.  It's of no value when you need a consistent AOA vs Chord reference.

"Shouldn't the upwash on an airfoil increase the effective angle of attack?" -- yes, it should, and it does.

absolutely useless answer if you dont care to explain......sorry

@quiet flyer can you explain your statement?

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text:
Why are highly cambered airfoils rarely used in propellers or wings?
Highly cambered airfoils like the Seligs and Epplers are hardly ever used in propeller or wing sections. Why?
Does their performance reduce at high Re? Are they only adequate at low Mach, low Re?
The question only regards the aerodynamics. Ignore fuel storage and structural elements.

First propeller use: A highly cambered airfoil would cause high pitching moments and twist the propeller blade. Of course you can pre-twist the blade so it will assume the correct shape in the desired operating point, but a propeller needs to work over a wide range of operating points, from take-off roll to high speed flight at altitude. In off-design points (i.e., most of the time) the propeller would have poor performance when compared with one which tolerates more diverse conditions.
Note that indeed thin, highly cambered airfoils are used on compressors and turbines in jet engines. Those are more stubby and enjoy much narrower variations in flow conditions, so the highly cambered, thin airfoil is indeed the best choice here.
Use on wings: Some aircraft do indeed use highly cambered airfoils. Those with low maximum speed like human powered or electric propulsion aircraft prefer those airfoils because they create the needed lift at the lowest possible speed, so the aircraft can fly with the limited installed power. As soon as the aircraft needs to cover a wider speed range, however, a lower camber is needed to keep drag low at high speed. This is similar to the use on propellers: A wider operating range requires to move away from the narrow optimum offered by those highly cambered airfoils.
Note that aircraft with a high wing loading use extensive and extensible high lift devices which turn their wings into thin, highly cambered structures for landing. Again, as soon as a lot of lift at low speed is needed, thin, highly cambered airfoils are the best choice. Adding slots between segments will make those work even better than a solid airfoil and allow to use more camber.

so in regards to propeller use, it is mostly a matter of load carrying capacity, twist and 'hot shape', correct? not because of its aerodynamics performance? Does it mean that if the stiffness of the blade was increased by geometry(increase chord) or by material (composites), then such airfoil would be ideal?

@toshi ba highly cambered airfoils are "one speed wonders".  Great to see them in jet turbine blades though, probably one of the reasons jets are most efficient near full power.

@RobertDiGiovanni Highly cambered airfoils behave much better in a duct where there is another airfoil surface "above" the suction surface to constrain the flow. Aside from biplanes and triplanes, that does not apply to wings, and ducted propellers are also a rarity except for special purposes.

@toshiba: If we hypothesise a super-stiff material and build a propeller from it, and fit every blade of it with camber flaps, yes, it could be beneficial at some operating point to activate those flaps. Most likely flap angle would then vary over prop span to improve twist for the actual advance ratio.

@alephzero check out lower speed applications and Reynolds numbers down to around 100,000.  For many aircraft and birds, highly cambered airfoils are the best thing going for L/D ratio.

use extensive high lift devices - I think you mean extensible.  I linked dictionary definitions of both, and retractable flaps (that's what you're talking about right?) are much more clearly described as "extensible", even if their area is extensive (large).

@PeterCordes: Yes, it means both. Extensible and large, comprehensive. After all, extensive comes from the Latin word for "stretching out" (extendere).

You can't just cherry-pick aerodynamics and exclude everything else when it comes to aircraft design. But let's entertain flight dynamics alone for this instance, and use Selig S1210 or S1223 as examples.
Selig-S1210 (for Re 0.2e6, 0.5e6 and 1.0e6) characteristics:

First, notice that the linear range of the airfoil extends from $C_l$ 0.5 to 1.9, which means that this airfoil is only suitable for low speed flight.
Second, the maneuverability of such a wing would be questionable. For a typical aircraft, we would need the ability to do at least a 0G push-over and a 2G pull-up (for Part 23 and 25 aircraft). However, the airfoil is clearly stalled before reaching $C_l$ of 0; and if we use $C_l$ 1.0 as design cruise lift coefficient, then 2G pull-up would also be difficult.
Therefore, while such high lift airfoil may serve niche applications such as high lift competitions with model airplanes, its aerodynamic utility is limited.

Delightful poster!  Here's a clearer version.

"The linear range of the airfoil extends from Cl 0.5 to 1.9, which means that this airfoil is only suitable for low speed flight". why?

@toshi Because you're forced to have a lift coefficient of 0.6 or more at cruise, which either means you'll fly slow, or you'll have a very large wing loading which is impractical for takeoff and landing.

forced by regulations?

@toshi No, but why would you want to fly in nonlinear range where the flow is separated, drag is high, and handling is nonlinear?

@toshiba - Forced by efficiency concerns

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text:
How accurate is XFoil / XFLR5 at 'kinda high' Reynolds numbers?
How accurate is XFoil/XFLR5 at Reynolds numbers between 2 million and 10 million?
From my understanding it's only accurate at lower Re values.

I believe it's quite the opposite: XFOIL would be more accurate at higher Reynolds number. The higher the Reynolds number, the smaller the displacement thickness and the closer the displacement body is to the original airfoil shape.

I edited a little and from the context I assumed that "2" meant "2 million"; if I got something wrong then you can just roll back my changes or edit again.

Before version 6.9 (30 Nov 2001), XFOIL had some problems at Re > 3 million, but that's now been fixed.  Its documentation says:

The BL equations are normally discretized with two-point central
  differencing (i.e. the Trapezoidal Scheme), which is second-order
  accurate, but only marginally stable.  In particular, it has problems
  with the relatively stiff shape parameter and lag equations at
  transition, where at high Reynolds number the shape parameter must
  change very rapidly.  Oscillations and overshoots in the shape parameter
  will occur with the Trapezoidal Scheme if the grid cannot resolve this
  rapid change.  To avoid this nasty behavior, upwinding must be introduced,
  resulting in the Backward Euler Scheme, which is very stable, but has
  only first-order accuracy.  Previous versions of XFOIL allowed a specific
  constant amount of upwinding to be user-specified.  Currently,
  XFOIL automatically introduces upwinding into the equations only
  in regions of rapid change (typically transition).  This ensures
  that the overall scheme is stable and as accurate as possible.


text:
Why does the European MALE RPAS UAV use twin engines?
I don't know why the European MALE RPS UAV designers chose to use twin engines?
I know two engines are expensive but add safety...
Does choice is to obtain mass flow @ Medium altitude(low air density)?
especially with relatively low speed(maybe 0.4~0.6 Mach)...
I guess that as density drops off at higher altitudes, the machine ingests less air mass, resulting in reduced power and reduced thrust...
To need larger flow areas (larger diameter)?
Unlike RQ UAV series...

Welcome to aviation.SE! I edited your question to be more specific, but if I got it wrong please feel free to edit again yourself. The [tour] might be helpful if you're new to StackExchange.

According to the Airbus press release:

The MALE RPAS is the first unmanned aerial system designed for flight
  in non-segregated airspace and will be capable of performing a range
  of missions in intelligence, surveillance and reconnaissance, both
  wide area and in theatre. A twin-turboprop propulsion system will
  deliver energy for the mission system and provide proper redundancy to
  limit restrictions when operating over European densely populated
  ground and unrestricted airspaces.

Essentially, the aircraft is intended to be operated in the same airspace as existing air traffic, over large urban areas. The twin engine configuration is to provide redundancy and thus stop a robot plane from crashing on people.

Three months after you've asked the question, the Wikipedia article you provided a link to in your question was updated to state:

The drone's dual engines were a demand of Germany, which intended to use to UAV for surveillance over domestic urban areas and was concerned that an engine failure in a single-engine drone could lead to the drone crushing a house. [sic]

Wikipedia states this article as the source for the German demand for redundancy.

Thanks @ymb1 for the edit, I did not consider the possibility that Wiki was updated after the original message. Noob mistake...


text:
How can aircraft land on choppy water?
What plane can land the slowest on the water? What plane can land on the choppiest water? Is there a way to land between the waves?

The answer to all 3 of those depends on the winds, in the end.

And timing... That's how the FAA suggests you do it anyway. Are you talking about seaplanes, or crashing?

The Grumman Albatross can operate in 4-5-foot swells. Is that the sort of thing you mean?

Depends on how bad is the sea state, but the general technique for seaplanes is to land parallel to the swell, preferably on top of a wave:

When landing on a swell system with large, widely
  spaced crests more than four times the length of the
  floats, the best landing heading parallels the crests and
  has the most favorable headwind component. In this
  situation, it makes little difference whether touchdown
  is on top of the crest or in the trough.

If forced to land against the swell, it is recommended to land on the backside of the wave:

If crosswind limits would be exceeded by landing parallel to the swell, landing perpendicular to the swell
  might be the only option. Landing in closely spaced
  swells less than four times the length of the floats
  should be considered an emergency procedure only,
  since damage or loss of the seaplane can be expected.
  If the distance between crests is less than half the length
  of the floats, the touchdown may be smooth, since the
  floats will always be supported by at least two waves,
  but expect severe motion and forces as the seaplane
  slows.
  A downswell landing on the back of the swell is preferred.

Could the anonymous downvoter explain if their issue is with me or the FAA handbook's recommendations?

Oh, now you're having issues too? It wasn't me, but there may be an additional procedure for "chop". Your answer is swell, though.

It may be "into the wind" with light chop, parallel with larger swell, generally not recommended in rough seas as a wing tip may catch etc.  What plane can land in the choppiest water may have a lot of opinions.  It is tough to find a good answer to a vague question.

@RobertDiGiovanni yes, I breezed right through it.

How high are the waves? Timing into the wind, proper flare and try not to capsized I reckon. Never sideways to the waves.

"Never sideways to the waves" I assume this means not parallel to the swell? I disagree, this is in fact the preferred direction to land a seaplane.

I should have said never sideways into the leading edge of the waves... Apologies


text:
How are this helicopter's blades perfectly timed with the camera shutter?
This is a video of the Mi-24 helicopter. The rotor blades appear not to be moving.
Did the pilot and photographer sync the camera and helicopter to make this video and how? I do not know the true source but the video is remarkable.

I think this should go on video.stackexchange.com

Closely related

Video is shot at fixed frame rate, so there's nothing the photographer can do to synch with the helicopter.

@user71659 I'm not convinced. The only answer a videographer could give is basically the comment I posted a moment ago, so a good answer (as has been posted) requires more knowledge about helicopters than about video.

@DavidRicherby Your comment made a mistake about video (which is why it should go in the other SE). While you have to come up with say, 30 images per second, the integration time, or shutter speed, can be much less than 1/30 second. When the shutter speed is << 1/30 s, you can vary its rate within the video frame (at the risk of introducing judder). This is done exactly avoid (or introduce) synchronization or to eliminate things like roll bars on CRTs.

@user71659 Heh, OK. The question has come up for reopening but I didn't vote either way.

@DavidRicherby no vote is better then a down vote. Thanks

We've seen this effect in film/video for years, but what makes this so striking is that newer digital sensors are so fast that they can have an extremely high shutter speed. On an older video camera the blades would just blur to a far greater extent.

If the rotor is spinning at 360 RPM, typical of helicopters, that's 6 revolutions per second, and 5 blades go by each revolution.  6 x 5 is 30, so you have 30 blades passing point X in any 1 second period of time.  Unlike film at 25fps, video is shot at 30 fps, in perfect sync with the blade passage, so the blades look stopped.  If the rotor was going a little faster or slower, the blades would start to move.
The video camera is in effect working like a stop action strobe camera.

Video cameras generate the frame rate from a quartz oscillator, that is very accurate. I think the interesting aspect of this question is, how/why the rotor keeps the spinning rate that accurate. Is is just coincidence, or could such a video be reproduced any time?

@bogl yes and the helicopter faces directly at the camera at one point leads me to think the pilot may have fine tuned the throttle to the camera.

The fuel control system on a turbine helicopter regulates a pre-set rotor RPM.  You apply throttle from idle to an operational setting that bring the RPM into the operating range, then fine tune the speed governor with blips of a toggle switch, setting the precise rotor RPM.  When you pull pitch with collective the governor system will add whatever fuel is required to keep that RPM.  That's why the RPM in the video is so consistent.

so it is on the helicopter not the camera making this question on topic?

Well.. on topic enough for me.  Some others are a bit more fussy.  For me, it involves a flying machine.  Good enough.


text:
What is the difference between a P-51 Mustang and an A-36 Apache?
Both were made by North American Aviation and the planes were almost twins - the A-36 having a ground attack role while the P-51 was a fighter. Are there any noticeable differences between the two?

You probably meant Piper (Apache).

They were largely the same aircraft but the first line of the A-36 wiki sums it up nicely

the ground-attack/dive bomber version of the North American P-51
  Mustang, from which it could be distinguished by the presence of
  rectangular, slatted dive brakes above and below the wings

later in the article they go on to list some more features

The contract for 500 A-36A aircraft fitted with bomb racks, dive
  brakes, and heavier-duty wing, was signed by Kelsey on 16 April
  1942,[6] even before the first flight of the first production P-51 in
  May 1942.

This website also lists the same mods as well as some other fun facts.
This website lists two other features noteably the vent window which is not mentioned elsewhere.

...and the installation of small vent windows in the windscreen side
  panels. In addition, two Browning M2 .50 caliber machine guns were
  mounted in each wing, and two more .50 cal. guns were mounted in the
  lower nose to fire through the propeller.

Please ... the A-36A has to have more wrong "information" on it both in print (as in books and magazines) and online in articles, photos and videos, than ANY other WWII aircraft.
First it is NOT a "derivative" or "version" of "variant" of the P-51A Mustang.  It's actually the other way around.  The P-51A is an offshoot of the A-36A - the last of the 500 A-36As rolled out of NAA Inglewood before the first P-51A was built.
Next, and THIS one is, for some reason, the most-repeated mistake of them all:  It's called the "Apache" in books, magazines, the Internet and on videos (and, as a result, in conversations between warbird nuts in person and in social media). The National Museum of the U. S. Air Force even had their A-36A "mislabeled" on the sign in front of it, until after I contacted them.  They corrected it to the only official name that it ever had, "Mustang," in September, 2018.
Here is an article that I wrote earlier this year.  Right after it was published, I received even MORE documentation from the Boeing Historical Archivist that, while it was superfluous (as in, "not really needed"), it verified beyond ANY shadow of a doubt, that "Mustang" is the only official name that the A-36A ever had.
Check it out: http://www.thehangardeck.com/news/2019/3/16/the-a-36-by-any-other-name
OH, and the only armament on the A-36A was the .50 Cal Browning Machine Gun - two were mounted in the nose/cheek position, and two in each wing. The 20 mm cannons were installed ONLY on the NA-91 aircraft, the first aircraft actually designated "P-51" (no suffix letters or numbers at first).  THEIR official name, in the USAAC/USAAF from late 1941 until 13 July, 1942 was "Apache."  After that date, anything made by NAA that even "looked like a Mustang" was called "Mustang," officially.  The A-36A came along AFTER that date in July, so it was a "Mustang" from "birth."
emphasized text strong text
If I knew HOW to attach images of documents to this post, I'd gladly do it.  The "Images" thing above seems to be only for images that are online, and my images are on my hard drives.
I will state that my documentation includes: scans of North American Aviation documents and newsletters and a Telegram from Dutch Kindelberger to Col Ennis, dated 13 July, '42, before the first A-36A was built;  references from the most-respected "Mustang Authors" (Robert Gruenhagen and Ray Wagner); references from the US Government.
I'm actually a humble person, so if someone would tell me how to attach MY images (without first getting them online, but directly from my HDDs), I'd be most appreciative.
My only goal here is to help my fellow warbird nuts be as correct as possible when talking about our beloved warbirds - REALLY

This would be a far better answer if it read less like a rant and more like wisdom from an expert.

Both this answer and the linked article (very interesting!) would be far better if they sited and linked or reproduced some of the sources for the claims. I'm not implying you're wrong (I wouldn't know it if you were), but if you're going to contradict "conventional wisdom" it's often considered helpful to show some evidence to backup your claims.

This doesn't actually answer the question asked but is instead what appears to be a personal attack on other answers/comments. The OP asks what are the differences between the P-51 and the A-36. Instead of deriding 'wrong information', how about specifying the differences between the two aircraft?

TJL, FreeManand and CGCampbell ... thank you for  your comments.
I did not intend that ANYONE would see this as a "rant."  In the limited time that we all have for our "hobbies," we cannot be singularly "devoted" to one point. I apologize if my writing style seemed to HINT that I was ranting.  I'm a retired hospital pharmacist and I can type as fast as I can think - it came from being in that profession since the early 1970s.
I have < 160 characters left in this "Comment" but if anyone is interested, I can EDIT the above and attach more documentation that what is really necessary.

@TomGriffith To add an images just click the "image" button in the editor's toolbar. A little window will appar in which you can simply drag-and-drop your local image or hit the "browse" button to locally browse for your images. Maybe you want to read the help page, section Images at the bottom.

The A-36 was a P-51A configured for ground attack with minor add-ons like the dive brakes and bomb racks (and 20mm cannon on some).  Both had the Allison engine which was not suitable for high altitudes.  The P-51 B and later had the RR Merlin.  Lots of other detail differences, but the engine is the main one when thinking of the A-36/P-51A vs P-51B and subs.

I'll try to NOT be a "rant" due to any perceived "sensitivity" on my part:  The P-51A was a derivative of the A-36A, if we're going to call the next aircraft the series a "derivative."  The A-36As finished production runs before the P-51As began to be manufactured by NAA.   It's even indicated in their NAA Model Numbers:  These numbers are consecutive, chronologically.  The A-36A Mustang was "NA-97" and that of the P-51A Mustang was "NA-99."
BTW, the "P-51A" designation is NOT a designation of all Allison Mustangs that were NOT A-36As.  It was the 5th production model of this "family."

@Tom Griffith, The A-36 is technically a variant of the P-51. It is heavily modified but remains extremely similar. Also, contradicting your claim, the first P-51A flew on October 26th, 1940. The A-36A apache, on the other hand, was first flown in 1942, meaning that the P-51 was actually built before. Also, have you ever heard the saying "outrageous claims need outrageous evidence"?, I recommend backing your answer with evidence.

I have a link to my article from almost 2 yrs ago at TheHangarDeck.com.  I shall try - TRY - to attach scans of images of Primary Source Documents that firmly establish the "Mustang" name for the A-36A, that are actually ADDITIONAL documents that substantiate my point that the A-36A was never called "Apache" officially - EVER - during the WWII years.  Certainly, the pilots and groundcrew of the A-36A in the 27th Fighter-Bomber Group serving in the MTO called the A-36A "Invader" and actually petitioned NAA to change the official name to "Invader," but THAT name was already given to Douglas Aircraft Company for their A-26 Attack/Light-Medium Bomber.  I've reread the above and I do NOT see anything insulting or condescending --- if anyone's offended, then I did NOTHING to try to do so.
Let me try to attach those two images - scans of an NAA document in the form of a Telegram directly from Dutch Kindelberger, HIMSELF, to the USAAF's Chief of Public Relations, and the cover of the Parts Catalog for the A-36A.
Note that the first A-36A was not built until 2 or 3 months after the date of this Telegram.  The first P-51A was not built until after the A-36As had been rolling out the doors at NAA Inglewood.  The last A-36As, the first P-51As and strangely enough, the first P-51Bs had production dates that overlapped each other. THAT is an interesting fact.

Left Side:  Telegram dated 13 July, 1942       Right Side:  Cover of A-36A Parts Catalog
(Both supplied by The Boeing Company, owner of both images and all credit due to them)

I agree with you about the fact that the A-36's official name is the Mustang. Some pilots and ground crews called it Invader, others even called it apache, but the official name is Mustang. I have changed the name of this question to "What is the difference between a p51 mustang and an a-36 mustang" instead of A-36 apache. However someone needs to review and approve it, and nobody has done that yet.

However you incorrectly stated that the first P-51 was not built until after A-36 were being built for some time. This is inaccurate because the first P-51 entered service with the RAF in April 1942, and the first flight of the A-36 Mustang wasn't until September 1942.

The A-36A's main differences from the P-51 or Mustang 1 are the dive breaks, reinforced wings, nose mounted browning 50 cals, and others. The A-36 is technically a variant of the P-51. It is heavily modified but remains extremely similar. Also, contradicting @Tom Griffith, the first P-51 flew on october 26th, 1940. The A-36A apache, on the other hand, was first flown in 1942, meaning that the P-51 was actually built before. The later models of the p-51 (B,C,D,k, H...) used the merlin engine, the apache however retained the allison.

J Lopez, et al.  I am "spread too thin" I guess, and I have not had a chance to come back to this website to talk about the A-36 story, the P-51A story and other more "general" Mustang stories.
I don't know why my works seem to seem like "rants."  The only thing I can say is that when some folks (such as I) state something that goes against the most-often-repeated "story" or "concept" or "idea" --- whatever --- it may sound like a rant.
The aircraft that  flew in Oct, 1940 was not a P-51 at all.  I was the concept aircraft built by NAA, to demonstrate to the RAF as a P-40 replacement.

@TomGriffith, the aircraft that flew in October 1940 was a prototype, there is a difference between a concept and a prototype. And that prototype was the P-51s prototype and was designed as a fighter. The A-36 prototype first flew in september 1942. The P-51 entered service with the RAF in April 1942. The A-36 entered service in June 1943, according to the national museum of the USAF, and many other websites. This proves that the P-51 was designed and built before the A-36, because it was first flown first, and entered service first.

J Lopez...I tried to send a reply, and this comment box said it was too long, even after I "trimmed" it to the point of ridiculousness.
I feel that YOU really want to know the facts - the truth as it were - about the designations and the order of production of the various production Mustangs.
I really want to help you with this.  I've done it wit a number of folks that I've met through Facebook.
Get back to me, please.   We have much to share and learn (I admit that I'm still learning after > 50 yrs of seriously LOVING Mustangs!

you're "splitting hairs."  I used the term "concept" a year and a half ago (I need to come here more often!) for this reason:  the NA-73X aircraft was built as a "civilian" aircraft - it had NO USAAC (or other government's) serial number.   Indeed, it had a CAA registration number, equivalent to a "Tail Number," but it was a CIVIL aircraft.
Once again - the Mustang Mk I that entered RAF service was NOT a "P-51" at all.  "P-51" was a USAAC/USAAF designation for the  third NAA aircraft of the Mustang family to be built, and the RAF's first Mustang is an earlier aircraft.


text:
Steep turn: do we need rudder once we rolled into a bank angle?
Say a Cessna 172 is in a cruise level flight and clearing turn is just finished. I barely feel a left-turning tendency during the level flight (Maybe torque and slipstream are less significant).
I understand that we should use the rudder simultaneously when rolling into a bank attitude (45 degrees-ish) and then add more power + apply back pressure on the elevator.
But once we've entered into a bank, do we still step on the rudder to stay coordinated? For example, I'm making a left steep turn. After I've rolled into a bank attitude and start turning, do I still need to step on the left rudder?

No.  You use the amount of rudder needed to keep the ball centered. No more no less.  Most of the rudder requirement is to correct for adverse yaw, and is only required while the aileron is being applied.
If you are in a steep turn, you may find you need to apply aileron out of the turn to hold the bank angle if the airplane has a tendency to "overbank".  You might need to apply "top rudder" in this case, but only because adverse yaw from the top aileron will be making the nose yaw toward the turn and the top rudder is needed to align the tail with the nose in the airflow, to keep the ball centered.
The muscle memory activity that needs to be ingrained is left aileron, left rudder and right aileron, right rudder, depending on how bad the adverse yaw is, taking out the rudder input with the removal of the aileron input, with adjustments as required.  Your feet should just be squeezing this way or that way to center the ball, period.
Elevator/power (and aileron) is the only tool for altitude/attitude control in the turn (by "and aileron" I mean that once bank gets really steep, top aileron is often necessary to prevent the elevator application from just steepening the bank and tightening the turn - if you are targeting a 45 deg bank, this should be instinctive and you will be applying top aileron as required without thinking about it).
Use of "top rudder" rudder to prevent the nose from dropping, as opposed to just keeping the ball centered, is a cross control maneuver that results in a slipping turn (the ball will move to the low side), and if you stall out in a slipping turn, you will roll over the top as the outer wing stalls first and start to spin.  Many pilots have been killed by using "top rudder" as attitude control while turning to final and finished their lives right there.
I should also note that you can use a slipping turn to lose altitude on the turn to final, so you are in fact cross controlled with into turn aileron and top rudder, but this is a special altitude loss maneuver where you are making no attempt to maintain pitch attitude or altitude, just letting the nose fall through.  Done properly, there is no stall risk when doing a slipping turn (you aren't pulling and in fact you might actually push a bit), as can happen if doing a level steep turn.
So don't overthink it.  Just use your feet to keep the ball centered at all times unless you are side slipping on purpose, and get yourself into the automatic habit of squeezing left rudder with left aileron input and right rudder with right aileron input, and no rudder with no aileron input (but making applications as necessary if the ball drifts off).  A 172 needs very little rudder application to maintain a coordinated turn so it should just be little squeezes of rudder.
If you learn to fly gliders, where LOTs of rudder is needed to do the same thing, these actions become well ingrained because you have to, to get the damn thing to fly straight at all, which is why I always recommended learning to fly in gliders first.

Great answer.  I might just suggest not making the elevator comment quite so absolute, and adding a short blurb about cross checking AOB when it comes to correcting for altitude deviations.  Because, for example, if you are targeting 45 deg, have steady backpressure but find yourself descending at 60 deg, you need to take out the bank before you increase the pull.  Or at least make both corrections simultaneously...

Yes agreed. Thanks for the comment.


text:
Is QNE pressure altitude?
In trying to understand pressure altitude better, am I wrong or right when I say this?
I am at an airfield. The pressure altitude at that airfield is 200 ft a.m.s.l.
If I set subscale 1013 mb on my altimeter, the altimeter should, theoretically, indicate 200 ft a.m.s.l.
Am I wrong or right when I say this? And am I right in thinking along these lines in trying to understand pressure altitude better?

If there is something you don't understand in the definition of QNH (e.g. as defined by wikipedia or skybrary), don't hesitate to precise it so that answers can be adapted to your current level of comprehension.

You can not edit the original question to make it ask an entirely different question all together. That is not the purpose of this site. It is not meant to be conversational. Please roll back your edit to its original form. If you wish to ask a brand new question, please use the “Ask Question” button to ask a new question.

Lungelo, I had to rollback your question to the previous version because you completely changed it. This invalidates the existing answer! You have to ask a new question for that.

That is not correct. There are three different altitude systems:
True altitude is the height above Mean Sea Level (MSL). This is what you get when you set your altimeter to QNH. Using this, your altimeter should read 200ft when you are on the runway if the airport elevation is listed as 200ft MSL.
Absolute altitude is the height Above Ground Level (AGL). This is what you get when you set your altimeter to QFE.  Using this, your altimeter should read 0ft when you are on the runway.
Pressure altitude has no relation to the ground or sea level. This is what you get when you set your altimeter to QNE (always 1013 or 29.92). You never use this near the surface of the Earth, only when up high enough that terrain or obstacles clearance isn't a concern. Pressure altitudes are expressed as Flight Levels (FL), with the last two digits omitted.


text:
What deviation from the official flight path is allowed during takeoff and landing?
Our local airport is currently going through CAP1616 and permanent airspace change consultations.
Considering take off and landing flight paths:

what degree of deviation is allowed from the official flight path by aircraft while under 7000 feet (2133 meters)?
what would be a typical distance travelled before reaching the aforementioned altitude?

A potential 3rd question relates to finding the designated flight paths but I believe this is answered in other questions.

Instrument flight and visual flights have very different standards and expectations regarding flight paths, and aircraft being guided in real time by ATC in a radar environment are different again.
For instrument path design, I believe what you are looking for is called a TERPS design standard(terminal procedures specifications). In the USA most of this is contained in FAA orders 8360.3 and 8360.19 the CAA, EASA or ICAO probably have some equivalent. However IFR path design mainly considers only ATC controlled vs uncontrolled airspace and restricted areas, not much weight is given to the various classes of controlled airspace. The different classes of controlled airspace exist primarily around ATC workloads.(traffic density) TERPS are mainly concerned with obstacle clearance, availible navigation and equipment failure contingencies.
This is not specific to the UK, but planes have the same physical limits everywhere so standards are similar. The industry standard final approach for instrument flights is 3 degrees or 318 feet per NM, steeper only when required for physical obstacle clearances. The obstacle clearance on approach generally changes from 1000ft in the initial segment to about 250feet at the missed approach point.(dependent on the exact form of navigation)
The standard minimum expected climb performance is 200ft/NM but the obstacle evaluation surfaces are only 152ft/NM (40:1) up to the en-route obstacle clearance. These climb slopes are reduced at high altitudes but that is not relevant to an airport in the UK.  Also EASA and the FAA have slightly different methods for setting the details, like where the slope starts, but the end result is similar. Normal climb performance in modern aircraft is usually above 400ft/NM not withstanding engine failure, or mechanical trouble like stuck flaps, lightly loaded jets can climb much steeper slopes.
As for lateral clearances, this is a bit complex and based on the type of navigation, a localizer/ILS final will have a wedge of plus or minus 3 degrees to either side of the center path and the wedge will be 700 feet wide at the runway threshold. This is usually up to about 1000ft above the airport. there are wide safety buffers either side of this 6degree wedge. With other forms of final guidance the path may be over a mile wide at its most narrow.
The initial segment and low routes are most often plus and minus 4NM from the center path, plus a 2NM obstacle safety buffer [12 miles total width].
WAAS(SBAS) enabled GPS nav can allow terminal area paths of plus or minus 1 mile from center-line(they still require a large safety buffer beyond this), reducing to 0.3miles on final and with some equipment a narrowing wedge similar to the affore mentioned LOC/ILS.

The lateral safety buffer for traditional enroute tapers from 1000feet altitude clearance over an obstacle at the edge of the primary air route(4nm from center) to zero feet altitude over obstacles at the outer edge(6nm from center)


text:
What is that wire connecting
What is that wire connecting the tail fin to the fuselage in beechcraft 1900 and so many other airplanes

It is an HF (High Frequency) antenna. It is used for aircraft to communicate on overseas flights. VHF (Very High Frequency) airband radios are used for normal local and cross country flights.
For a radio to transmit effectively, the internal components of the radio and the external (to the radio) antennae both have to match the frequency used for the transmission. If either does not match, the signal will not go out as strong nor as far. The way the antennae match is by having an antenna that corresponds to the wavelength of the signal. The way you determine the length based on the signal frequency is by dividing 234 by the frequency in megahertz. This gives you your antennae length in feet. Therefore, the higher the frequency, the shorter the antenna needed.
For this next section, we will relate the radio wavelength to energy. The higher the frequency, the shorter the wavelength. In our examples, the shorter the wavelength, the more “energy” the radio wave (or signal) has.
The reason this is important is that radio signals travel in straight lines. They are just like light. They require a lot of energy to punch through obstacles. If the radio receiver is in the radio “shadow” of an obstacle, it will not receive the signal. Since the Earth is curved, this limits the distance a particular radio wave can be received from its source. The line-of-sight (straight line seeing) distance of an observer standing on the Earth’s surface in flat terrain is only 60 miles.
One way to combat this is to use lower frequencies. Frequencies of less than 30 MHz do not have enough of the required energy to punch through the protective obstacle surrounding the Earth called the ionosphere. Instead, the signal gets reflected back down towards the Earth where it gets reflected back up again. The signal will then bounce between sky and Earth for as long as it has energy to travel. Sort of like a billiard ball.
When traveling extremely long distances, like overseas, you have to utilize this signal bouncing feature of HF radios to reach your intended recipient. Radios in the UHF and VHF range can only be used as line-of-sight transmitters because, at frequencies above 30 MHz, the wavelength of their signals will punch through the ionosphere and not get reflected back down to Earth.
P.S.
HF antennae can be bent or folded to accommodate less space. The antenna length can even be divided in half or fourth, but may induce frequency matching errors. Dividing it by eighth will compound those errors too much to make it useable in many cases. Any other division will make the antennae not match.

It’s a high-frequency antenna, used mostly on flights traveling across seas and oceans, where there tends to be areas without VHF and UHF coverage. This was the case on older aircraft, most newer ones have a smaller hidden one on the tail-fin.


text:
What's the meaning of "the break is a level" in a recovery operation of an aircraft carrier?

The break is a level, 180° turn made at 800 feet (240 m), descending to 600 feet (180 m) when established downwind. Landing gear/flaps are lowered, and landing checks are completed.

(source: wikimili.com)

The "break" is where the aircraft enters the landing pattern. The aircraft carrier tries to time entries into the break such that aircraft are landing in 15-20 second intervals. Until you "break" you are in a holding pattern around the aircraft carrier. Entering the "break" is getting into the pattern to land.
When they say "the break is level" it means the altitude is maintained during the turn to enter the pattern, so it is a "level turn".
Here is a graphic that illustrates the level turn, then the descent when established on downwind.

Source: Wikipedia

"Level" and "descending" seem contradictory.

Addressing the above and the attempted edit... It isn't ambiguous. The descent isn't done until established on the downwind. Until you are established you are turning cross-wind. The turn is not descending.

So you think the descending movement isn't part of the turn. The author has made an awful sentence expressing the meaning. The break is comprised of a level turn and a descending movement?

@Allan Yes... I've added a graphic that illustrates my point.

"Level" isn't a noun here--it's not that the break is a level.  Level is an adjective describing the kind of a turn.  A break is a turn; that turn is a level turn (neither descending nor ascending), and a 180 degree turn, and it is made at 800 feet.

In other words, "level, 180° turn" is the same construction a "big, brown dog" or "fast, red car". Both "level" and "180°" qualify "turn".

Sorry, I copied and pasted it carelessly. I think I'd better not edit the title since there's an answer about my carelessness.

The break is a 180° turn from the upwind to the downwind in the landing pattern which allows for the separation of elements in a formation and spacing of individual aircraft for recovery.  Typically the break (for a Case 1 recovery) occurs on the upwind leg at 800 ft AGL, 1 NM DME ahead of the ship, at 350 kts.  The lead aircraft breaks formation, turning to enter the downwind with his wingman continuing on in the upwind, making the turn to the downwind 15-18 seconds later.  This allows for an approx. 45 second spacing between aircraft in the downwind and final approach legs to ensure spacing for each jet to recover, taxi clear of the LA, and reset the arresting gear for the next jet.


text:
Why do all fifth generation fighters (J20, F22, SU57) use Trapezoid wings?
It seems that trapezoid wings are used by all fifth generation fighters (although the wing of J20 looks quite like delta wing), but why? What made a trapezoid wing so attractive? Why don't they use other forms, like the swept wing on SU27?

Trapezoidal wings have advantages and disadvantages:

Better performance than delta wings at transonic speed
Good stealth performances
Flaps can be easily used.
drag reducing
better Lift/Drag ratio than delta wings

The problem is that they stall at much lower angles of attack than delta wings.
Edit:this problem is overcome using leading edge,LERX,and/or canards

I’ve edited your question a bit.  If I accidentally removed something important, feel free to edit or revert to the original.

Thanks for your answer! But can you compare the trapezoid wing with the swept wing(like ones on SU27) ?

what is biscus drag?

drag caused by layers of the fluid sticking to the object.More surface area means more of the object is in contact with the fluid, which means more drag

possible typo in above comments -- the word is "viscous"

Yes it's viscous drag....(because of automatic corrector)


text:
What does a white diagonal line mean on an Airbus ECAM DU?
When my A321 powers up, the lower ECAM DU shows only a diagonal white line. What does it mean and what would be the fix?

You have a 321, but lost the POH? And don't have a service contract?

Seriously? If you're a licensed engineer you'd already be consulting Airbus's approved service manuals or the Airbus service team. If you're not a licensed engineer you ask someone who is. A stranger on the internet might tell you to hit it with a hammer. Would you do that? (Hint: don't!)

I'm not sure why people are voting this question down: it's clear and presumably there's some sort of answer to it. Saying that "you should be able to answer this yourself" isn't particularly helpful. As for the 'don't trust what strangers on the internet say' comment, that's perfectly true but you could say the same thing about every question on this site :-)

I never could understand why people downvote a question when the OP has a rep of 1. It seems kinda pointless, sort of in the same vein as beating a dead horse.

@Pondlife The answer is to simply follow the procedure in the A320 TSM, which the OP hasn't provided any evidence of following, or even knowledge thereof. It isn't a useful question to anybody to post a list of AMM procedures for every random aircraft fault.

@pondlife The problem with Stack Exchange engaging with this sort of maintenance question is that it's a safety issue. There will be an approved procedure documented for this particular aircraft that might differ from an apparently very similar aircraft operated by another airline. Getting it wrong could be seriously damaging to the health of hundreds of people. I wouldn't want to fly with an airline whose principle source of maintenance information is 'some bloke on the internet'.

@user71659 Sure, and IMO this is one of many grey areas that we have on this site, which is normal. Is it a useful question or not? Is it a safe question or not? I don't know the answers to those, but I don't like closing a question only because we 'disapprove' of it. Anyway, I'm just one person with an opinion and this is why we have votes :-)

@ReddHerring That's all very true, but is it our job to 'police' questions, identify ones with safety of flight aspects, and then close them? For example, we've answered lots of questions about flying technique and procedures in light GA aircraft and getting it wrong in those cases could also have serious consequences. What's the difference?

@pondlife The difference is the relationship between the person asking the question and those most affected by an error. For GA-related queries this is often the same person, who can make a judgement on whether he/she consider the answers appropriate. For airlines, those with most to lose are the passengers, whose safety depends on the integrity of the certification and licensing system. One 'engineer' with a cavalier approach to proper procedure can wreak havoc. 'Some bloke on the internet' doesn't have a place here and we should not enable it. This might be a good question for Meta.

The question likely comes from someone using a serious game simulator, it seems such questions are downvoted and closed with a vengeance by some of us. So just ignore them and upvote...

@ReddHerring Personally, I don't like making assumptions about the background to questions. mins has a great point that this could be a sim, for example. Or it's a question from some kind of maintenance written test that the person doesn't understand. There are lots of plausible possibilities that don't involve putting real people at risk. But it could indeed be a good meta question and if you want to discuss it then don't hesitate to ask it there!

Thanks to all that responded. As for Redd Herring or whoever this superior engineer is, why be negative?  It’s a valid question and I guess when you work on aircraft you have a laptop by your side to access the maintenance manuals at all times?  I did not and have only been an engineer for 48 years. I subscribe to the philosophy that there are NO STUPID QUESTIONS.

Display Management Computer issue:

If a DMC fails, all information is removed from the displays it
  controls and replaced by a white diagonal line to indicate a working
  display with a failed source. ECAM will detect the failure and provide
  a sequence of alerts.
Due to the priority rules, the E/WD display will
  move to the lower DU if it was DMC1 that failed. The EIS DMC switch is
  used to replace the failed DMC with DMC3.

Source

Steps to clear warning:

First select DMC #3 to isolate fault to DU or DMC. If DMC is at fault
  reset above associated circuit breaker for 5 seconds. If fault is with
  DU reset these (CAPT PFD/ND = G3/E1) (FO's PFD/ND = R3/R4) (Upper ECAM
  = E5) (Lower ECAM = R11).

Source

More in the maintenance manual at AMM31-60-00-710-001. (EIS start).


text:
What are the pros and cons of wingtip tanks?
I am looking into the aerodynamic pros and cons of wingtip tanks. Let's assume they're perfectly streamlined, optimally placed and sized for a given airframe.
I am aware of the purpose of fuel storage on a wing that's too thin or otherwise inadequate for 'wet wing' fuel storage and as a means of permanent fuel storage, but how do tip tanks interact with airflow on the end of the wing?
What are the pros? Are tip tanks effective as 'winglets' or drag-reducing tips, controlling spill and cleaning up vortices? Do they contribute to useful lift and if so what percentage and what is the overall effect on the wing?
And what are the cons, aerodynamic, structural and general? Do they require heavy modifications to the spar or wing in general to attach?
Overall, if a wing design can include bladder or wet wing storage, would it be better not to employ tip tanks?

Your question has a "2.", but no "1.". Did something accidentally get deleted, or did you just change formatting mid-stream? You can [edit] your question to fix things up to make your intent more clear.

Pro: they look sexy.  Con: the runway wants to have sex with them.

I happen to own a homebuilt with tip tanks for ALL of the fuel supply (Pazmany PL-2), and while they look really cool, I'd prefer the fuel was in the wing leading edges mid span.

There is no aerodynamic benefit.  The tuna fish shaped tank doesn't function like a winglet (it has to be a wing to work like a winglet) and is too small to have any kind of end plate effect of significance (an end plate has to extend for a full chord to do any good that exceeds its own drag).  The flattened and canted tanks you see on twin engine Cessnas was intended to produce a bit of vertical lift and an enhanced dihedral effect to offset some of the inertial issues in roll that the fuel mass creates; which leads to...
Inertial issues in roll.  The mass of the fuel way outboard in the wings is a total pain.  It deadens the aileron response.  The inertia magnifies roll and yaw disturbances. My airplane is much more snappy with minimum fuel vs full fuel, and wags its tail in turbulence much less. I prefer to fly it with about 1/2 fuel if I'm just going out for an hour or so.  The  mass also kills a lot of the roll-yaw couple and amplifies adverse yaw.  If I skid the airplane with rudder, the roll into the yaw is very mild, in spite of significant dihedral.
Significant form drag. On my plane there is about a 8kt speed penalty from having the tanks taking up space in the airstream (based on the performance of other PL-2s modified to tankless wet-wing).
Fuel imbalances have a huge impact because the arm is so long.  If I ran one tank all the way down with the other nearly full, I'm sure it would use up an unpleasant amount of my aileron authority at low speed (I don't really want to find out how bad - at minimum, it would take probably 10lbs plus of side pressure on the stick to keep wings level). Having no trim tab or bungee, I use the fuel balance as my lateral trim, and when solo, have to achieve about a 2-3 gal right side differential to get the plane to fly hands off.  Then I have to remember to switch back and forth every 10 minutes or so to keep the balance.  I know if I forget because the airplane starts to wander off toward the heavy side, like a horse that suddenly remembers where the barn is while you're riding it.
Structural issues are minor.  You are adding mass at the opposite end of the most highly stressed part of the wing, but you need to make the outer spar structure able to handle the local bending, so some minor reinforcement would be required at the attachment.
One plus is the fuel supply is far away from you, say if you flipped over.  I like that.
One minus is the fuel supply is far away from you, at the first point to touch the ground in any kind of landing excitement.

All that being said, the PL-2 is a delightful airplane even with the tip tanks, and dammit they look awesome.  If I was building one from scratch however, I would do leading edge tanks like the Van's RVs.  Just way more efficient in every way.  As a long range fuel supply add-on however, they can make sense, but I think most designers will just extend the wet wing volume instead, until they run out of that.
On airplanes like the Lear, they made sense because with the super thin wings they were the most practical way to put in a large fuel supply.  But you will notice that they did away with them in later models because other than a handy place to stick tanks that don't get in the way of something else, it's mostly negatives. I shudder to think about how to control a tip-tank Lear with one side nearly empty and one side nearly full, if that were to happen somehow.
A former colleague was an old F-101 Voodoo driver who also had a lot of time in T-33s and he recounted to me once that the T-33's really large tip tanks were a pain due to the flywheel effect of the fuel mass. A little bump would start it rolling, and with the big weights out there, it took more than the normal input to stop the roll and if the conditions were rough you were fighting that constantly.

Thank you for describing the behavior with full and empty tip tanks. While clear in theory, it is good to get it confirmed by practical experience. One question, though: Are aerobatic limits reduced with empty tanks?

Why did you decide to build/buy an aircraft with wingtip tanks?  Did you build it yourself?

If the plane has the tanks designed-in from the start, it provides a degree of span loading and the root structure can be lightened. This can save more weight than strengthening the tip adds. Big tanks can also reduce wing losses, allowing a smaller area and offsetting some of the drag. But big ailerons will always be needed to give normal roll rate.

@RockPaperLizard my PL-2 was built in the 70s from plans by the original owner. The behaviour I describe is not bad enough to want to so something radical like re-do the fuel system; it's just not really optimal and I would do it differently if I was building it.  I knew what it would be like having read pilot reports long ago of old Cessna 310s with very similar tip tanks.

@GuyInchbald yes the Paz's ailerons are quite large and a bit heavy by homebuilt standards but are very crisp and you fly the plane with just fingertip pressures.  The span is 27 ft so the roll rate isn't that great even with little fuel in the tips although more responsive.

@PeterKämpf no there are no fuel limitations for aerobatics I know of other than the fact that the design aerobatic gross weight is lower than the max gross. My plane is aerobatics prohibited under local regulation so I don't have an aerobatic GW with my wb documentation, but I recall it's something like 1350 lbs and my max gross is 1500. The wing is one piece with 27ft long spar caps, custom made at one time by Alcoa for Pazmany to sell to builders. He redesigned the spar to a built-up spar the builder could make after builders complained about the 800 dollar cost(in 1970, about $4000 today).

@PeterKämpf actually the most interesting aspect of this airplane is the surprisingly weak roll/yaw couple, kind of glider like.  There is a normal amount of dihedral and I would expect a good skid to produce enough roll rate to make the plane reasonably controllable on rudder alone.  But it just kind of goes sideways and very slowly rolls to the low wing.  You wouldn't want to try to control the plane close to the ground on rudder alone.  Fortunately, the aileron control linkage has a fail-safe design insofar as you can have a rod end disconnect at any one location and still have one aileron.

Thank you for the extensive information! The page you linked says that the PL-1 had even less dihedral. As a low wing design it should have weak yaw-induced roll and those 5° are quite normal – the Me-109 had 5.75°.

That makes sense.  I learned to fly in the mid 70s in a Piper Cherokee, which has 7 degrees and that and the low aspect ratio would give it a bit more roll rate from skidding.  PL-2 does start to roll if you skid it, eventually, but there is a big delay and I think the fuel mass is the cause of the delay.  Lateral stability wise, it seems perfectly adequate and remains wings level quite nicely once in lateral trim, in smooth air.  Tail wag in bumps is noticeable and it seems to lack fixed fin area.  A lot of them have dorsal fins added.

I just know that it forms a vertical partition which prevents the air flow from passing from the lower surface (overpressure) to the upper surface (depression). On some gliders, this partition also serves as a "shoe" to protect the fin when the wing is on the ground. can be used on some aircraft as additional tanks. Fouga-Magister, Cessna 320, Learjet 23, etc...and it seems to me that the weight at the end of the wing compensates for the upward bending effort due to the lift.


text:
How does the 737 MAX MCAS differ from stab trim in other 737 variants?
I have the following questions:

MCAS will not cut out from excessive control column movement as non-MAX 737 aircraft stab trim does. Is that correct, and is this cutout permanent or does it  re-engage after the control column forces/movement are relieved?
MCAS can adjust the stabilizer to its hard stops whereas non-MAX 737 aircraft have soft limits. Is that correct?
MCAS operates the trim wheel motor in approximately 10 second bursts with 5 second pauses. What do non-MAX 737 aircraft do?

You might want to break this up into separate questions lest your post get down voted for being too broad.

Normal and non-normal Stab Trim operation is the same for all B737 variants. Only the B737 Max has MCAS and therefore it’s operating characteristics really can’t be compared to the other variants.

@MikeSowsun Of course they can: both STS and MCAS adjust the same flight control surface.


text:
In what emergency situations would a pilot bail out of a helicopter?
The Hiller UH-12 POH actually mentions “Emergency Exit in Flight” as an emergency procedure. What emergencies would make the pilot do this and why don’t other helicopter POH’s mention this?

This is exactly why no helicopters use parachute any more.

Erin, what helicopters are you familiar with where the crew routinely wear parachutes?  I flew them in the 80's and 90's and never wore one.

@KorvinStarmast the UH-12 manual linked mentions unbuckling the parachute harness when ditching, so apparently parachutes were worn at the time.

I fly a light airplane very often, with no parachute, and have never felt the need of one. And I'm not alone in doing this. In the airfield where I have my plane, I have never seen a parachute...

@jwenting were Hillers typically flown above 1000' agl?

@KorvinStarmast I don't know. But that's what the flight manual says to do in case of ditching :) May well have SOP for pilots to wear parachutes in military aircraft at the time, and it just carried over despite being useless, who knows.

@jwenting Ditching is flying or landing the helicopter into the water and then getting out.  Bailing out means you abandon the aircraft while it is flying, and you use a parachute.  If you aren't at least 500' AGL you are usually dead on impact before the chute deploys.  Depends on the chute and how quickly you get the D ring pulled.

@KorvinStarmast irrelevant. I know what ditching is :) Fact is, the POH says to detach the parachute harness prior to ditching, indicating that one was used. That's the only relevant bit here, not the difference between ditching and bailing out.

One example of a helicopter bailout that I am aware of was the ditching of a USAF Sikorsky HH-60G during the infamous "Perfect Storm." After several failed attempts to refuel from a tanker in the extreme weather the pilot decided to ditch. He hovered to allow the other occupants to bail out, but he was unable to bail out himself as the engines flamed out from fuel exhaustion and he autorotated to the water. After it hit the waves it rolled over. At this point he realized that he had not jettisoned his door as per the procedure for bailout or ditching so he was trapped. He managed to escape, but one of the other crew was lost at sea.

Tom, did the 'bail out' include a parachute?

@KorvinStarmast No parachutes. Most of the crew were rescue swimmers. They jumped from low altitude. But the waves were so high that if they didn't time it right at the crest they fell like 40 feet to the water.


text:
What is the meaning of "regular pitch" when applied to a propeller?
I am looking at parameters of a propeller that looks like this:

3 blade xx inch diameter (regular pitch)

What does "regular pitch" mean?

Could you show us the propeller you are looking at so we can see this in context?

This is not a common expression?

Context is going to be key here. Can you post an example where you've seen this phrase?

Ill assume you are asking the context of fixed pitch propellers. In which case "regular pitch" is either the standard pitch for the airframe in question or some mid range pitch to deliver average performance. There are usually various propeller options for a given aircraft:

There are two types of fixed-pitch propellers: climb and cruise.
  Whether the airplane has a climb or cruise propeller installed depends
  upon its intended use. The climb propeller has a lower pitch,
  therefore less drag. Less drag results in higher rpm and more
  horsepower capability, which increases performance during takeoffs and
  climbs, but decreases performance during cruising flight.

You can read more here.


text:
Do UL aircraft complying with LTF-UL need to copy an existing design pattern?
With my current pipe dream being to get physically involved in aviation
instead of only theoretically, I looked into the regulations for ultralight planes in Germany.

On multiple occasions in the LTF-UL regulation it is mentioned that in special cases the design inspection authority "Musterprüfstelle" needs to approve the aircraft.
This and the fact that most articles about building your own UL are referring to using existing design patterns "Muster" makes me wonder if it is even possible/feasible to design your own UL aircraft.

So the question is split into two parts:

Is it possible to legally design, build and fly your UL in Germany (complying with LTF-UL)?
Is it feasible to design, build and fly your UL in regards to the financial and time-killing aspect of the bureaucracy in contrast to using an existing design pattern?

Not an answer, but you might be more interested in experimental than ultralight aircraft. Have you considered that venue?

@aCVn That of course would be more fitting but to my research I couldn't find an "experimental aircraft" class in the German regulations.
I only know of "motorisierte Luftsportgeräte" (motorized aviation sports equipment, MTOW <600kg).

I'm not familiar with German regulations specifically; a separate question asking whether German regulations have provisions for experimental aircraft in general, and what the relevant regulations would be in that case, might not be unreasonable. That would allow this question to focus specifically on UL aircraft.

@aCVn  I asked the question about experimental aircraft regulation in Germany

Your starting point for regulations about certification of aircraft in Germany is the Luftverkehrs-Zulassungs-Ordnung (LuftVZO).
§1 Zulassungspflicht states

(3) Ein Luftfahrtgerät, dessen Nachbau nicht vorgesehen ist, wird als Einzelstück zugelassen. Einzelstücke sind von der Musterzulassung befreit.

So no, there is no Musterprüfung/-zulassung anymore, but still an Einzelstückzulassung.
How does this work? Let's take a look at the Verordnung zur Prüfung von Luftfahrtgerät (LuftGerPV)

§ 3 Einzelstückprüfung
(1) Der Nachweis der Lufttüchtigkeit eines Luftfahrtgeräts nach § 1 Absatz 3 der Luftverkehrs-Zulassungs-Ordnung wird in einer Einzelstückprüfung erbracht, deren Art und Umfang von der nach § 2 zuständigen Stelle festgelegt wird. Das Gleiche gilt für Änderungen, die sich auf die Lufttüchtigkeit des Luftfahrtgeräts auswirken. Die zuständige Stelle kann Dritte mit der Überwachung der Prüfung beauftragen.
(2) [...]
(3) Absatz 2 gilt nicht für Luftsportgerät. Die Verkehrszulassung von Einzelstücken eines Luftsportgeräts wird in der Kategorie „Luftsportgerät“ erteilt.

The competent authorities (zuständige Stellen) in the case of Luftsportgerät/Ultraleicht are the Luftsportgerätebüro im DAeC and the Deutscher Ultraleichtflugverband, where you can apply for an Einzelzulassung.
So yes, you can design and build your own ultralight, and it doesn't have to be similar to an existing design, but its airworthiness still has to be assured, so it can be quite an involved process.
More details on the process can be found in this report of college students at the HAW Hamburg.


text:
Is an EASA flight school required to give me my training records if I leave?
I am having troubles with my flight school in Austria/Europe. So I want to change school but the school won't give me my training records so I can't finish my training somewhere else.
Is there any EASA regulation that states they have to return my records?

Have you asked them to transfer the records straight to another school? I had a similar issue here in Australia where I was not allowed the records about me but they were happy to send them to another school.


text:
Could helicopters use a rear wing to get more lift from the rotor vortex?
Could a wing be added to the rear of a helicopter to catch the vortex from a helicopter for more lift? How would that affect transitional lift while in forward flight?

Source: https://www.danubewings.com/translational-lift/
How do modern helicopters tackle Vortex Ring State?

Yes, in forward flight only and with questionable effectiveness, since the rotor would need to compensate for the pitch down moment. Note some helicopters have a small horizontal stabilizer on the tail boom, that is mostly used to pitch up and keep the airframe level in forward flight as seen here (look at the camber of the stabilizer):

In all other flight regimes the wing would be a liability as it would interfere with the airflow downstream of the rotor.
It would also require significant structural reinforcement of the tail boom to transfer the bending moment of the wing, likely large enough to compromise the VTOL capabilities.
In summary, yes, but it would make no sense, there are better places to place a wing on a helicopter.


text:
How can I calculate which runway to use for landing, based on the wind?
I have a project where the user enters the name of the airport. We fetch the wind direction (degrees) for that airport. Then we use the wind direction and the data about the runways at that airport to calculate which runways the planes will land at. What is the best way to do that?
My approach:
Find the compass direction, for instance 90 degrees is equivalent to east. Then, if the direction of the wind is east, the plane will land from the west (to land against the wind).
My question:
How do I calculate which runway the plane will use to land? There are multiple runways at some airports. Do we use runway heading, or runway longitude/latitude, assuming wind is the only factor affecting landing? Is there a mathematical way to calculate which runway the plane will use? I am asking because I will have to do some calculation to accurately give that answer to the user.
Any expert/aviation help would be greatly appreciated.
Also, I am using the airports and runways table from ourairports.com/data.
Also, what do HE and LE mean in the context of runways?

Closely related, maybe a dupe?

The name of a runway is a number indicating its direction in tens of degrees.1 For example, to land west-to-east, use runway 09.
Generally, each runway paving may be used in either direction, and the physical paving actually contains two logical runways, running in opposite directions. Only one of these logical runways is allowed to be in use an any time.2
The two logical runways on a paving have names that differ by 18. Thus a paving aligned due east-west will contain logical runway 09 and logical runway 27.
The information you will need about each airport is simply a list of the available runways and their lengths. You will also need the minimum runway length required for your aircraft to land.
To determine the best runway to land on, divide by the wind direction by ten and use the runway that is long enough whose name is closest to the result.
Example: An airport has runways 04 (1200'), 09 (5000'), 15 (5000'), 22 (1200'), 27 (5000'), and 33 (5000').3 Your airplane needs 3500' to land. The wind direction is from 232 deg.4
So, 232 divided by ten is 23. Runways that are long enough for your aircraft are 09, 15, 27, and 33. Of these, the one closest to 23 is 27. You would request and expect landing runway 27.5
This is a rough outline of the steps a math project would take to guess the landing runway. Most pilots have no trouble hearing the weather advisory, glancing at the Jepp chart, and just knowing which runway should be in use.

Often two digits are used, i.e. runway 9 is called 09.
If a tower controller is present, he will identify and grant access to the runway in use. Your project will calculate the most likely expected runway. With no controller, the pilot should co-ordinate with any present traffic before using your project result.
Notice that reciprocal runways occupy the same paving and perforce have the same length.
Your project must be told whether the reported wind direction is from or toward. When the reported wind direction is toward, as it might be from a non-navigational source such as a local news broadcast, you must calculate the reciprocal from direction.
If you had a plane that could stop in 1200' you would use runway 22.

Reported wind direction is the direction the wind is blowing FROM.  So you don't need to do the reciprocal. With wind direction of 282, you'd use runway 27.

After removing any runways that aren't long enough, wide enough or paved enough for the aircraft in question, of course, and whether there is a usable instrument approach if needed. And assuming no parallels or local factors (e.g. terrain, airspace) that can't be solved algorithmically

@Gerry: Thank you -- I missed the sentence "We fetch the wind direction (degrees) for that airport." - Whether the wind is announced is the direction TO or the direction FROM depends on the source, but that sentence does suggest a nautical or navigational source, which will be FROM.

I did understand some things. But just to understand better, in my project there is no issue regarding runway length. Also, each runway has 2 numbers, 1 on each end. Do I use both numbers of each runway to calculate the difference between wind direction and runway direction.

Another questuon,

What does HE and LE mean?

@Faizan: Please re-read the second and third paragraph in my answer, where I explain about the two runways occupying the same physical space, but in opposite directions.

Thanks Breveleri,


text:
Where can we use conditional clearance?
Besides Line up conditional clearance in a PREPARATION FOR TAKE-OFF situation, where else can we use conditional clearance (on an apron, taxiway, etc.)?

I am interested in ICAO STANDARD.

Welcome to Aviation! Please feel free to [edit] your question to indicate the ICAO standards detail - not everyone will read the comments before answering, and comments can get deleted. While you're at it, please take the [tour] and read through the [help] so you can make the best of this Q&A web site!

Conditional clearances can be issued for any stage of flight, but are usually not as they increase risk outside of the areas that you have listed.
In the en route environment a controller can issue conditional climb instructions for example:
SIA1223 at DOGET climb to FL340
BAW2283 passing FL250 track direct DROGO
I have never issued a clearance like that and some contries have made them not available in their regulations. But they do exist in other phases of flight.

Under FAA rules a conditional LUAW may not be issued, but any other conditional may be: "Pass behind traffic" (on the ground or in the air), "reaching 5000, turn left heading," "At FIXXX descend and maintain," etc.


text:
Why do Dassault Falcons only have one thrust reverser?
Using all 3 engines would slow it down much faster. No?

Because it doesn't need to slow down faster.

Not a dupe, but this airliners.net thread may help

@kris what the...???

@Jpe61 agreed. "drop like a ball"? The plane is already on the ground...... By the way, can someone comment on the "high temperature/pressure difference" part? He might be onto something, but again, turbofan exhausts are supposed to not be very hot or high pressure

There is no reason the fuselage would crack or split because of thrust reversers. There is some huge misunderstanding here...

As always, it is a trade-off.
On the positive side, thrust reversers allow you to slow down faster and reduce brake wear.
On the negative side, there are costs to install and maintain them. They add weight, which increases the required take-off length reduces the maximum payload and increases fuel consumption.
Dassault found the optimum for their design in one thrust reverser, others go with TR on all engines, none or optional.

"On the negative side" they is also a risk of deployment in flight which needs careful mitigation and expensive certification demonstration.


text:
Is it legal and safe to take off or land in zero visibility?
Is it legal for planes to takeoff and land in zero visibility? Is it safe?

Why the downvote, downvoter? This is a solid question and one which CFIIs cover during training.

In the US, for general aviation flights, operating under Part 91 of the FARs, it is legal to take off with zero visibility (see page 2-2 of the IPH, PDF). In my opinion, it is not safe. If an emergency were to take place, you would be unable to return and land; under Part 91, it is not legal to land unless conditions are equal to or better than a runway's published approach minimums.
Unless you've got a great alternate nearby, conditions are improving rapidly, or you've got an extra engine or two, you probably shouldn't take off in conditions you couldn't land in.

You might want to clarify what "part 91" means in this context, it's not obvious to many people.

Good call. Fixed some wording and added links to 91 and the Instrument Procedures Handbook.

The update is good, but what I was really thinking is that "part 91" is US only, so for many readers it might be clearer to just refer to "general aviation" instead?

This is definitely a case of "What is safe is not always legal ; What is legal is not always safe!" -- There's a reason most pilots won't opt for a "zero-zero" takeoff. My instructors always drilled into my head that you don't take off unless you're darn sure you can turn around and land again in you get into an emergency situation. (And the stuff I fly in isn't equipped for Category IIIc ILS approaches :-)

@Pondlife Actually it would be interesting if someone from another country/region could chime in and tell us if they have different requirements there (like "You can't take off unless you could legally make an approach to the field you just left")

@Pondlife rephrased to "for general aviation flights", which I agree is more useful. Also, I'd love to see non-US answers to this question.

Yes, some non-US answers would be great and they would really help us get a grip on the whole question of country-specific answers and regulations

IMO, a zero-zero takeoff in a twin would be more dangerous than in a single.  Loss of control on/just above the runway if you lose an engine on takeoff is busy enough when you can see fine.  With no visibility it's going to be a very ugly event.  I'd rather glide into a tree in a dead single.  :-)

Also, Cat III ILS approach minimums are close to zero visibility (75m or so), and is what some people call zero visibility.

@xpda, while true, most GA pilots can't fly Cat III approaches. I tried to make my answer as relevant as possible to GA. I don't see why there wouldn't be room for an additional answer about airline operations, though.

I hadn't gotten the hours I needed flying for the US Navy to take off with 0 visibility and 0 ceiling.  I asked an old timer why he had it and he said that, "They gave it to me because they new I would never use it."

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text:
When are flight tracks publicly available?
Is there a way to make sure a flight is tracked on flightaware.com or similar sites?
Similarly, is there a way to prevent a flight's track from being available to the public?

Skimming FlightAware's FAQs:

The most consistent way to show up on FlightAware is to have your flight worked by ATC (a squawk code and an IFR flight plan), or to fly an airplane equipped with ADS-B Out.
FlightAware doesn't really track VFR flights unless they have ADS-B Out:

some VFR aircraft with flight following are available on the position maps but it largely unreliable

  You can block your tail number in a few ways. Both the FAA and FlightAware operate block lists.

If you are operating VFR with flight following, you might be able to explicitly request the controller input your VFR flight following into in the "N A S" (National Airspace System)  http://discussions.flightaware.com/post144279.html#p144279

That whole FAQ page is pretty interesting.

Sites that track aircraft including FAA 5 minute delayed are Flight Radar 24 and Plane Finder I am also putting up a private tracking network in the Philippines.
These sites use volunteers that host equipment on an internet connection. Typically these sites will receive Data from Mode S Transponders from up to 200+ miles away with the right aerials. I only need about 6-8 sites to cover the whole Philippines. Line Of Sight limits below horizon reception, so you do need a station close to an airport if you want to get ground traffic.
If you emit Mode-S then you will be seen. There are no regulations to prohibit that and almost impossible to enforce.
Anyone can create this setup for less than $30.00, so there is no point in worrying about who is seeing what.
The FAA so have the ability to hide a specific aircraft, you can request that, but the private networks will report the data.
There is another technology called MultiLateration that can determine an aircraft's location form 4 or more ground stations with Mode A or Mode C transponders. Flight Radar are actively implementing this in Europe now. It requires a network called PlanePlotter to operate.

To be visible in the FAA data, you must still be operating under an IFR flight plan (or sometimes VFR flight following). Even with multilateration, Mode A and Mode C transponders don't transmit a unique aircraft identifier (only temporary squawk codes) so you will still need to have a flight plan open with ATC to be uniquely trackable.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
When should a transponder be used?
When should a transponder be turned turn on?

On the ground?
Always in flight?
Only when required: during radar service (flight following), above 10,000ft MSL, inside the Mode C Veil, or on IFR flight?

Should Altitude Mode be used whenever the transpoder is on?

At least in the United States the FAA recently (I think in 2012?) changed the recommendation for transponder operation in the AIM.  It now reads:

Civil and military transponders should be turned to the “on" or normal altitude reporting position prior to moving on the airport surface to ensure the aircraft is visible to ATC surveillance systems. IN ALL CASES, WHILE IN CONTROLLED AIRSPACE EACH PILOT OPERATING AN AIRCRAFT EQUIPPED WITH AN OPERABLE ATC TRANSPONDER MAINTAINED IN ACCORDANCE WITH 14 CFR SECTION 91.413 MUST OPERATE THE TRANSPONDER, INCLUDING MODE C IF INSTALLED, ON THE APPROPRIATE CODE OR AS ASSIGNED BY ATC. IN CLASS G AIRSPACE, THE TRANSPONDER SHOULD BE OPERATING WHILE AIRBORNE UNLESS OTHERWISE REQUESTED BY ATC.

(Cribbed from Chapter 4-1 of the AIM)
So, basically "Turn it on before you taxi, and turn it off right before you shut down the engine. Always use the Altitude mode (Mode C) unless ATC tells you otherwise."

In practice having the transponder on while on the airport surface doesn't do much unless your field has something like ASDE-X which plots aircraft transponders on a screen in the tower, but it generally doesn't hurt anything and it ensures you don't forget to turn the transponder on when you take off.
Operating your transponder while on the ground may cause "ground clutter" on nearby radar station's displays (possibly setting off the conflict-alert system) - this is why many of us were taught to set our transponders to standby mode while on the ground. In practice this usually isn't an issue, and if it is a problem local ATC will ask you to "squawk standby while taxiing" to resolve the issue.
If anyone gives you a hard time about it copy the number and politely quote the AIM.

the other reason why we were taught to turn the xponder off on the ground is because older transponders had a cavity magnetron, which is a kind of high-voltage tube. The idea was to turn them off when not in use, to prevent them from wearing out, and to reduce the load on the electrical systems in old airplanes

@rbp the standby mode runs the cavity tube heater (at least it does in the AT-150 I have), it just doesn't connect it to transmit replies when the transponder gets interrogated. Whether you do more damage cycling the heater on & off or letting it run all the time is debatable, but turning it off on your taxi back to the ramp does take about an amp off the electrical load which is something if your electrical system is marginal at low RPM.

If the above quotation is from the AIM, then it is recommended, not required (as you've noted). Note: I'm not asserting that it's not good practice to use ALT reporting when taxiing! Is there a FAR # that mandates any transponder use outside class A, C and the class B 30NM veil?

@CJBS FAR 91.215 would be the base regulation: Paragraph C says: While in the airspace as specified in paragraph (b) of this section or in all controlled airspace, each person operating an aircraft equipped with an operable ATC transponder maintained in accordance with §91.413 of this part shall operate the transponder, including Mode C equipment if installed, and shall reply on the appropriate code or as assigned by ATC. (Bolding mine - "required everywhere except Class G airspace" is my read of that regulation.)

@voretaq7 Thanks - good reference. I'm assuming 'airspace' doesn't include taxiing on the ramp or even taxiways (per the FAR), given that the ground isn't controlled airspace.

@CJBS As far as I can tell it's not a regulatory requirement on the ground (at least under Part 91 - Parts 121 & 135 may impose additional requirements). At larger airports with ground surveillance radar it may be an operational requirement though (i.e. ATC will ask you to turn on your transponder so they can see you on their displays).

In the past it was recommended to turn off the transponder on the ground. I believe this was because a bunch of planes on the ground would show up on controllers' displays and make it hard to see the airborne planes. Now, filtering makes that a non-issue, and they recommend transponders on the ground so planes can be tracked for tower/ground controllers.

I'd like to add that it also allows ATC to verify mode C readout by comparing against field elevation. So they don't have to ask "Say altitude passing"

For aircraft equipped with ADS-B Out, the following regulation applies (emphasis mine)

14 CFR 91.225(f)Each person operating an aircraft equipped with ADS-B Out must operate this equipment in the transmit mode at all times

This a nuance to the ADS-B regulations that many may miss. So, for ADS-B it is required to be on (ALT) even while on the ground.


text:
When sharing an airplane, what liability does the owner bear?
What liabilities apply to the owner of an aircraft who allows other pilots to use his aircraft for not-for-profit and non-commercial use?
Example: An aircraft owner has a friend that wants to use his aircraft on a semi-regular basis (20-40 hours/year). His friend is willing to pay a fair share of maintenance, hangar, fuel, etc.
(Liabilities not limited to insurance, but also regulation, maintenance, etc)

Most of the liabilities (but not all) fall under the insurance of the aircraft. Your insurance policy is your largest liability. This is important not just for protection of your aircraft, but if your insurance ends up void in an incident, you could have a lawsuit on your hands. Don't assume that aviation insurance is similar to car insurance, either.
Insurance

Pilot (on plan)- Aviation insurance has 3 catagories: Pilot, Named Individuals, Other (explained later). The owner pays insurance based on their qualifications and those specifically named. If you are going to be borrowing on a regular basis, weigh the cost of adding you to the plan.
Pilot (off plan) - Insurance specifies whether or not you may borrow the aircraft to others, but also what qualifications that pilot must have. You may have only have 100 hours and are insured, but the insurance company may ask for 500 hours of someone you borrow the aircraft to.
Aircraft - Regardless of the pilot, if the aircraft is out of compliance on maintenance or airworthiness, it is not being flown legally and therefore voids the insurance. As the owner (primary on the insurance), this is your responsibility as an owner and the pilot's responsibility as PIC. Both of you can be hit for this one.
Reimbursement - Just because it is non-commercial use, your insurance may still specify what you can be compensated for. This may be only the actual expenses of the flight (fuel, oil, airport fees) and may include limits on hourly charges for maintenance. Keep in mind that this does not authorize you to break regulation on commercial use. You cannot make a profit.
Legal flight - Above all else, the flight must be legal. The flight needs to comply with all FAA regulations.
Non-commercial use - I keep coming back to this, but this is very important. The pilot may be a commercial pilot, but this doesn't authorize them to use the aircraft for hire. With many insurance plans, you can have a CFI train you since you are the owner, but a CFI cannot train someone else and you cannot train someone else. This makes and aircraft for hire and requires a 100-hour inspection.
Other - Pilots who do not own an aircraft may have their own (renters) policy. While this may cover liabilities of those in the aircraft, don't assume that this also covers your aircraft. A pilot paying $200/year won't likely have coverage for your shiny new Cirrus.

Other
As the owner, you have liability in accurate record keeping. If you fudge documents on your aircraft that make it appear FAA compliant to the PIC, then you can be held liable.
Notes
These are only the items that come to mind. Feel free to add.


text:
Where can I find nice animations of flutter?
Where can I find nice flutter animations/videos (other than YouTube) to add to a presentation without violating Copyright regulations ? It can either be for wings as well as blade arrays.
Are you aware of any OpenSource database on this topic ?

YouTube is really your best bet - is there any reason you wouldn't be able to use one of the many NASA videos?
If you need a release for legal reasons NASA is famously liberal about granting rights to their research/educational videos - just contact then and you'll almost certainly get an official "Sure, you can use it!"

The link is dead due to the youtube account being terminated.

This play list is going to be enough https://youtu.be/5ZTK6kEtHag